Scattering (S-)parameters (the elements of a scattering matrix) describe the electrical behavior of linear electrical networks and devices when undergoing various steady state stimuli by electrical signals. The parameters are useful for electrical engineering, electronics engineering, and communication systems design, including microwave engineering. Many electrical properties of networks of components (inductors, capacitors, resistors) may be expressed using S-parameters, including gain, return loss, voltage standing wave ratio (VSWR), reflection coefficient and amplifier stability. Although applicable at any frequency, S-parameters are mostly used for networks operating at radio frequency (RF) and microwave frequencies where signal power and energy considerations are more easily quantified than currents and voltages. S-parameters change with the measurement frequency, so frequency must be specified for any S-parameter measurements stated, in addition to the characteristic impedance or system impedance.
Vector network analyzers can be used to measure S-parameters, and vary in the number of ports usable to connect with a network or device. The network or device itself can have multiple ports. Vector network analyzers designed for simultaneously measuring the S-parameters of networks or devices with more than two ports are feasible but quickly become prohibitively complex and expensive. Commonly, a single port measurement of the input port voltage reflection coefficient is useful. Combining two single port vector reflectometers can provide flexibility and potentially reduce costs where single port measurements are more commonly obtained, but where multi-port measurements can be beneficial as well.